Generally, it has been known that exhaust gas discharged from internal combustion engines contains particulate matter (PM) mainly composed of carbon, ash made of noncombustible components, or the like and causes air pollution. Therefore, restrictions on the discharge amount of the particulate matter have been strengthened year by year, besides restrictions on the discharge amount of components such as hydrocarbon (HC), carbon oxide (CO), and nitrogen oxide (NOx) contained in exhaust gas. To this end, technologies for collecting the particulate matter from exhaust gas and removing the same have been proposed.
For example, a particulate filter for collecting the particulate matter is provided inside the exhaust path of an internal combustion engine. For example, since a gasoline engine discharges a certain amount of particulate matter together with exhaust gas although the amount of the particulate matter is smaller than that discharged from a diesel engine, there is a case that a gasoline particulate filter (GPF) is installed in an exhaust path. As such a particulate filter, a so-called wall-flow-structured particulate filter has been known in which a base material is constituted by a multiplicity of porous cells and the entrances and exits of the multiplicity of cells are alternately closed (Patent Literature 1 and 2). In a wall-flow-type particulate filter, exhaust gas flowing in from the entrances of cells passes through the wall of partitioned porous cells and is discharged to the exits of the cells. Then, particulate matter is collected by pores inside the partition wall while the exhaust gas passes through the wall of the porous cells.